Why aren't trams/trains autonomous?

Some are. Retrofitting infrastructure necessary to facilitate reliable, safe autonomy in all conditions is expensive and building systems to meet expectations in all conditions is also.

Many railways grapple with issues of reliability and accuracy despite being partially or fully automatic.

With cars, you have the easy out that the responsibility always rests on the driver. If you mess up, it's their fault; their hands should always have been on the wheel. If you break it, one person is late for work. If you break a rail network, everyone is late for work. Different risk profiles.

Being able to ship code of dubious quality and iterate on it is preferred for companies rather than engaging in a massive contract to deliver a perfectly working rail inventory and routing management system.

There's no practical difference between 'tram", "train" and "light rail".

There are lots of autonomous trains all over the world. Many airports use them. Many city subways use them.

You probably haven't been looking hard enough. They are there. The first time you ride in one can be a little unnerving when you can look out through the front window and there's no driver blocking your view.

Here the current underground trains are almost fully autonomous. They still carry a person in the cabin "just in case" and mostly because of union pressure. There's also a plan where new trains to be delivered in 2025 will be fully autonomous and have no driver at all.

AFAIK the situation is similar in some other European cities.